Reduce housekeeping executions in `Projects::AfterImportService`

In gitlab-com/gl-infra/scalability#20, we saw excessive garbage collection from pull mirrors that hadn't been changed in months.

Should Projects::AfterImportService run only once after the initial import? Import by repository URL doesn't run this for every repository mirror (https://gitlab.com/gitlab-org/gitlab-foss/blob/1bab0ba591263cd739af2d2c7c3f1b03678a59b6/app/models/project_import_state.rb#L53), yet projects imported from GitHub/Bitbucket/etc do.

Should we at least make this consistent?

Ideally, we'd only run HousekeepingService if there were any changes to the repository. I think this requires some help from the Gitaly RPC. git doesn't return any special code if the repository was changed, so we can only tell by the stdout results.